Method for inductive heating and monitoring

ABSTRACT

A method for transverse flux induction heating using a heating machine including an iron core, that essentially has a U-shape with an opening facing downwards. The legs lie in contact with downwards facing displaceable core jaws. These are so arranged that they are displaceable towards and from each other by means of pneumatic pistons. The object that is to be heated is placed between the core jaws. The core jaws are at the displacement guided by means of U-shaped rails and rolls or linear bearings. The displacement serves among other things to free the object that is to be heated or has been heated for transport into and out from the machine. When the right position has been reached the displacement is stopped and the two core jaws are locked by a pressurized bellow pressing the core jaws upwards against the legs of the U-shaped core. In this way a locking and a securing of as good contact as possible is obtained between the moveable core jaws and the U-shaped core so that the field is hindered as little as possible. Monitoring of heat parameters is done using one or more cameras.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of inductiveheating and in particular transverse flux induction heating. The objectof the invention is to provide a more efficient and precise method ofcontrolling the heating of an object.

One source of difficulties with inductive heating is that the propertiesand structure of the heating machinery will inevitably change with time.The powerful magnetic fields in the core will magnetize the differentparts. Because the field is alternating the parts will possibly vibraterelative to each other. Over time, this means mechanical wear on thecore. The ends of the core close to the object being heated becomefrayed causing changes in the field at and through the object. The partof the core close to the object being heated also becomes hot. Thiscontributes to changes in the core and thus the magnetic fielddistribution. If the objects that are to be heated differ, for instanceextrusion tools with different aluminum profiles, this also changes theheating circumstances.

For instance, when heating extrusion tools made of metals and alloys, inparticular aluminum, an increased precision in controlling the heatingis desired. These tools, before their use, must be heated in order toallow the aluminum to be pressed through the tool. Otherwise thealuminum “freezes solid” and the tool might break. Conventionally, thetools are heated for many hours or days in an oven before the extrusiontool is mounted in the extruding machine. In the beginning of theextrusion process, neither the quality nor the quantity that is obtainedfor the extruded profiles is acceptable and one must wait a while beforegood quality and full production speed can be achieved. This occurs whenthe surface of the tool that is in contact with the aluminum has arrivedat essentially the same temperature as the heated aluminum. Also, if atool is inserted too cold, the risk that it breaks is great, becauseheat tensions may arise and the pressure from the press is very large.

SUMMARY OF THE INVENTION

In view of these problems, it is the object of the present invention toprovide a method of inductive heating for an object and the monitoringof the heating, thereby dealing with the aforementioned issues.

In accordance with this invention, the problems with wear on the heatingmachinery are dealt with by monitoring the heating and adjusting it, ifneeded. The heating parameters may be altered by: changing the coretopography at the surface facing the object to be heated; or by alteringthe relative positions of the core and the object to be heated; orcombinations thereof so for instance a non-uniform heating can becounteracted. Also, the current fed to the magnetizing coils can beadjusted.

Additionally, the invention undertakes the heating of an extrusion toolbefore its mounting in the press by means of induction whereby the localtemperature distribution may become very close to what exists during thecontinuous extruding process.

It also diminishes the time before the quality of the extrusion profileis acceptable and limits the loss due to rejection at the beginning ofthe process. Therefore, frequent changes of the tools are possible, ifdesired, with only small storage requirements. Overall, this method is agood use of expensive extruding presses.

Moreover, the tools are not overheated by this process. This is a greatbenefit because in some cases, the peripheral parts do not have to beheated to the same temperature as the rest of the object. In othersituations, the inner working surface of the tools does not need to beheated to its full working temperature until shortly before it is put touse. Uniform heating would occur if an oven was used, but this does notnecessarily occur with this invention. This is advantageous because atthe working temperature the air could easily corrode the tools.

The heat monitoring is accomplished by means of a heat camera. Forexample, this method can be used to find the proper profile for heatingof an extrusion tool. First, an extrusion tool that was just used islifted into the heating device and the heat camera(s) register(s) theheat picture. This picture is then stored. Next time that tool needs tobe used, it is heated as close as possible to the previous heat pictureor the proper heat picture is extrapolated for when the tool isfunctioning properly.

Also, another application is crimp fittings (railroad wheels on axles,for instance). The controllable heating in accordance with the inventionmay find use in the hardening of cogwheels, cog, etc. because itminimizes use of effort and time, and reduces the strain and temperatureinfluence on treated objects due to non-uniform heating. Similarly, forball bearing races, the heating can be controlled to achieve a uniformheating.

In accordance with this invention, the control of the heat generationmay, for instance, be achieved by the use of specific adaptors betweenthe object that is to be heated and the magnetic core, thereby closingthe magnetic field in the heating device. Alternatively, the adaptationmethod used can be varied with time. This can be done, for instance, byswitching the adaptation means. By varying the times of the differentadaptation means, it is possible to gain good control of the heatingprocess.

In particular, the core or core jaw may include an elongated part thatcorresponds to an opening or channel in the core or core jaw. Thischannel is displaceable and extendable for the insertion intodepressions of a heated object or can be slid through this if theopening in the heated object is large. For instance circular objects,such as ball bearing races, may be uniformly heated in this way, becausethe circular objects function as a secondary winding in a transformer.Because the current heating of the object is the same all the wayaround, the heating will be uniform. Even several races may be heatedsimultaneously by being fed or replaced one at a time in the heatinglocation. By this method, the heating time may be multiplied by thenumber of races that are heated simultaneously. This can increase thefeed speed. By monitoring only the races that have been there thelongest and that are to be removed at the next feed interruption, thetime or field strength can be controlled to give precisely the righttemperature or heating conditions. Also, a high production rate ispossible without sacrificing the time that the heated object has toremain heated in order to obtain the intended molecular reorganizationbefore, for instance, cooling at hardening.

A further degree of freedom in the heating is obtained by varying thestrength and frequency of the magnetic field. A lower frequency resultsin a deeper heating while a higher frequency results in a moresuperficial heating.

It is also possible to use magnetic shielding to reduce heating ofspecific parts or objects by screening or shortcircuiting of the field.

Further advantages and characteristics of the invention are apparentfrom the patent claims and the following description of an embodimentdescribed with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus used in the method.

FIG. 2 is an adaptation piece that can be used with this invention.

DETAILED DESCRIPTION

The heating machine shown in FIG. 1 includes an iron core 1 closingmagnetic flux loops. The core has an essentially U-shape design with theopening facing downwards. Against the downwards facing ends of the legs2 and 3 are the displaceable core jaws 4, 5. These are so arranged thatthey are displaceable towards and away from each other by means ofpneumatic pistons 6, 7. Between the core jaws the object 19 that is tobe heated is placed. The core jaws are displaced towards and away fromeach other guided by means of U-shaped rails 8, 9 that run on rollersarranged in the frame of the machine. (As an alternative one can uselinear bearings.) The displacement serves partly to free the object thatis to be heated or has been heated for transport into or out from themachine. Also with this mobility, the core jaws can be placed thecorrect relative distance from the object required to achieve thecorrect heating conditions. When the correct position has been reachedthe displacement is stopped and the two core jaws 4 and 5 are locked byeach being pressed up against the legs 2, 3 of the U-shaped core 1 bypressurized air from a bellow means 12. In this way, the object islocked in place and the best possible contact is obtained between themoveable core jaws and the U-shaped core, so that the field is hinderedas little as possible.

The object that is to be heated can be inserted between the moveablecore parts on a moveable roll table or the like. The object that is tobe heated can control the heating, and be moveable laterally andheightwise, as well as circular, especially if it is a round object.

On each side of the object that is to be heated, a coil 13 is arrangedaround each core jaw. The coils are connected to a current source.

On each side of the object that is to be heated heat cameras 21 arearranged at the side of the coils for monitoring of the heating progressof the object. For instance, two, or alternatively, four cameras may bearranged on each side of the object and distributed over thecircumference and turned obliquely in towards each other. Fiber opticsmay be used to allow viewing by the cameras more or less through thecore jaws and adaption pieces.

The moveable core parts are in their front ends each provided with twohorizontally, laterally projecting pins 14. On these pins adaptionpieces 15, 16 can be hung for the adaption to different objects that areto be heated. These adaption pieces are provided with panels or flanges17 that can grip over the core jaws. The flanges 17 of the adaptionpieces are provided with V-shaped downwards facing recesses 18. When theadaption pieces 15, 16 are hung over the pins 14 they can grip overthese pins. The oblique edges can slide on the pins pulling the adaptionpieces against the core jaws. In this way, the adaption piece whenmounting will automatically be pulled against the respective core pieceso that good contact is achieved even if the movement precision ismoderate. On the upper side or somewhere else, the adaption pieces maybe provided with eyes or other gripping means. The lifting and mountingcan be executed by hand or by means of a robot.

Before heating, the workpiece that is to be heated is first pushed intothe heating area on a carriage (not shown) that is insulated from thecore. Thereafter the two adaption pieces are mounted from above. Whenthese are in place the core jaws are extended forward until the desiredair gap or contact exists between the object that is to be heated andthe adaption pieces of the core jaws. When this position has beenreached, the jaws are locked by being pressed against the U-shaped ironcore. Then, the coils are fed with AC of suitable amperage and frequencyin order to achieve the heating that is desired.

During the heating the heat cameras monitor the progress of the heating,and the registered temperatures are compared with previously recordedtemperature curves for the same tool. If the tool has not previouslybeen used, a temperature curve from some similar tool can be used.Preferably, the monitoring is done with a computer (not shown) that mayaccess temperature curves and other parameters stored for each tool sothat one only has to indicate what tool it is to enable the computer tocontrol the heating progress. If the adaption pieces need to be changedthe current is switched off and the adaption pieces are exchanged,either automatically or by a person. Alternatively or in addition theposition of the heated object may be adjusted or changed. If so desired,each tool may in order to reduce the risk of mistakes be provided withidentification markings that can be read by suitable reading means.Starting from previously recorded temperature curves and temperaturepicture sequences, the heating time can be adapted to what is needed.Preferably, the computer may signal if too great or specific types ofdeviation from previous heatings are registered because this mayindicate either that the wrong record has been accessed by the computer,or the wrong tool inserted, or that the tool has damage in the shape ofcracks. For instance, the tool may have developed cracks at previous usebut they have remained unnoticed because they are thin and difficult tosee.

Additionally, by disrupting the current to the coils and withdrawing thecore jaws or the dismounting of the adaption pieces, it is possible forthe cameras to get pictures or a sequence of pictures of the entireheated surface, even during heating when the view may be partlyobstructed. These disruptions need only take a few seconds and thuscontribute only negligibly to the heating time. Instead, the gainedinformation reduces the heating time. By letting the cameras registerthe progress of the heating while disrupting the heating only slightly amodel of how the heat transport take place in the heated object isobtained. Based on this knowledge, it may be calculated how long thecontinued heating should take place.

The heating process can be adapted to each occasion because of the heatmonitoring ability and adaption pieces available in this method. Forinstance, it is possible with the invented device to heat the tools thatare used in the extruding of aluminum to almost the exact workingtemperature in advance and eliminate the waiting time for temperaturebalance to be established after extruding for a time with the rightspeed. During the extruding of aluminum, a temperature gradient willdevelop in the tool. The parts in contact with the hot aluminum will bethe hottest and then the temperature falls away from the contact point.In the optimal case, the tool is heated to the same temperaturerelationship that exists during prior uses of the tool in the machine(with a possible addition for the cooling during mounting). In this waythe risk of a tool breaking is minimized. Also, if the tool should breakbecause of the heat tensions arising from heating this breakage willhopefully occur already during the heating before the inserting into themachine. Also energy consumption is minimized. This may not necessarilymean that the heating is to take place entirely from the center but onecan very well imagine that at first a major part of the tool is heatedand that then after a quick change of adaption pieces the central partsare heated to the final temperature, all in order to secure that thetools are subjected to as small strain as possible during heating aswell as during use.

A further embodiment of the invention is when the core jaws are hollowedand the core jaws' moveable core pieces are arranged inside, forinstance, to heat surfaces recessed in the workpiece. This is notunusual in connection with aluminum extraction tools. These inner corepieces may use the same method as used for the core jaws, that is, belifted into contact with the surface located above for the purpose oflocking and elimination of air gap. By bringing the different core partsinto contact before the electric current is closed, the risk of excesspressure and the exertion of forces on the guides are eliminated;therefore longer life is achieved for the core parts. In cases where theworkpieces that are to be heated are not too thick, the adaption piecesmay be profiled in the depth direction.

Another embodiment is the use of several parallel core pieces that whenpositioned correctly are compressed by the lifting movement. The openingin the core, for instance, can be turned 90 degrees in relation to theouter profile so that a package of several core bars can be pressedtogether and locked. If using bars of the same length, viewing andadjusting the bars is easier. In the extreme case, several smallseparate core bars could be moved individually into contact beforelocking.

In the same way as the core jaws, the core pieces may be provided withmountable adaption pieces, that are possibly retractable together withthe core pieces in the core jaws. In this case, it is not alwayspossible to mount the adaption pieces before the workpiece is insertedbetween the core jaws, then if necessary they could be forced intoposition.

By lifting the core jaws up against the U-shaped core before themagnetic field is generated, the risk of excess pressure is eliminated.Also, the risk of deformation of the core parts is eliminated.

The device according to the invention can also be arranged to providenon-uniform heating of other objects, for instance railroad wheels.Railroad wheels need to be heated in the center in order to be crimpedon an axle. Also, cogwheels can be heated along the outer edge forhardening of the cogs.

The heating can locally be distributed with different adaption pieces orby movement of the heated object. Furthermore, the heating of differentparts may be distributed in time so that the desired result is achieved.Preferably the sequence of adjustments and adaption pieces isregistered. As realized the invented device as well as the method arewell suited for automatic work.

The invention is, further, very well suited for use in production linesfor multiple products, for instance crimp mounting, hardening where inthis way increased precision can be obtained and giving in turn a moreuniform quality.

The use of adaption pieces provides not only an adaption of the magneticfield but provides also a protection for the actual core parts of themachine so that it does not have to come in direct contact with theheated object. By this interface, the heat transfer to the core jaws ishindered. The receiving area for the workpiece and the core jaws arescreened from the machine by means of a panel 20 of stainless steel.Since the adaption pieces 15, 16 are larger than the openings for thecore jaws in the panel 29, essentially, the heat radiation past thepanel and out towards the coils is eliminated.

In the above described embodiment, the U-shaped core has its openingfacing straight downwards. Other arrangements are of course alsopossible and in particular one can consider an arrangement where thecore is inclined for adaption to workpieces that lean against a supporton the transport carriage. By having both the core jaws moveableadvantages are gained when dealing with heavy workpieces. The workpiecethat is heated can continue to lean against its support on the carriageso that the core jaws do not have to take up the weight. By arrangingthe gripping points for lifting appropriately on the adaption meansthese may automatically be given the right angle for the mounting.

Electrically driven screws can be used instead of pneumatic pistons forthe positioning of the core jaws and the central moveable pieces.

What is claimed is:
 1. A method for inductive heating which comprisesthe steps of: inserting an object to be heated between the ends of thelegs of an essentially U-shaped core with at least one coil of wirewrapped around the core's jaws; locking said object in place between thelegs; feeding said coils alternating current of suitable amperage andfrequency to achieve inductive heating with a magnetic field in saidobject; monitoring heat by one or more heat registering cameras andcontrolling said heating by adjustment heating parameters or conditionsuntil an intended temperature condition has been obtained for theobject; and adjusting heat transfer in the object by changing a positionof a displaceable core jaw attached to said legs and extendable coilinserts attached to either the core or the core jaws.
 2. A methodaccording to claim 1, wherein heat transfer to said object is controlledby changing adapters connected to either the core jaws or the legs.
 3. Amethod according to claim 1, wherein several objects are heatedconcurrently and a next one of the objects that is scheduled to beremoved is monitored and heating is controlled to provide apredetermined condition for said object.
 4. A method according to claim1, wherein several objects in a group are heated concurrently and a nextpair of said group that are scheduled to be removed are monitored; thenext object to be removed of said pair is maintained at a suitabletemperature by controlling magnetic field around said object, andheating is reduced to maintain the second object of said pair at anintended temperature once a proper temperature has been reached.
 5. Amethod according to claim 1, further comprising the step of heating thecenter of an extrusion tool to a higher temperature than area around theouter edge of said tool.
 6. A method according to claim 1, wherein atool after it is used for extrusion is placed in the heating area andits heating profile is registered with the cameras and stored; the toolis monitored for a sufficient time to allow an interpolation of itsinner temperature; and the tool is heated again, the heating beingcontrolled to produce a similar heating profile.
 7. A device forinductive heating comprising an essentially U-shaped core with legs ofsaid core against two core jaws; said core jaws being displaceable fromeach other and being lockable in position around an object to be heated;at least one coil being wrapped around said core for handling suitableamperage and frequency needed to achieve inductive heating; anextendable coil insert attached to either the core or the core jaws; andone or more cameras are arranged around the core.
 8. A device forinductive heating according to claim 7, wherein said core jaw on theside facing said object to be heated has mountable adaptation means forcontrolling magnetic flux in the vicinity of said object.
 9. A devicefor inductive heating according to claim 7, and further includingadaption pieces having flanges or panels that can grip over the corejaws laterally, said panels or flanges being provided with downwardsfacing recesses with oblique side edges that can grip over pinsprojecting laterally out from the core jaws so that said adaption piecescan be mounted from above and by their own weight be drawn against thecore jaws when slid downwards along the oblique edge of said pins.
 10. Adevice for inductive heating according to claim 7, and further includingmoveable core pieces for accommodating objects with differenttopological surface structures.